Fluxonium: An Alternative Qubit Platform for High-Fidelity Operations

Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit coherence, fast frequency tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, within the realm of superconducting qubits, reveals an alternative qubit platform that is competitive with the transmon system.

Figure 1

(a) False-colored optical image of the fluxonium processor made of aluminum (colored and white) on a silicon substrate (black). The colored metals correspond to the circuit components in the schematic shown in (b). The two qubits $Q_A$ (red) and $Q_B$ (purple), each consisting of a superconducting loop made from Josephson junctions [25] and a shunting capacitor, couple to each other through direct capacitive coupling. Individually, qubits are driven through capacitively coupled microwave charge lines (blue), frequency control is achieved through inductively coupled flux lines (orange), and readout is achieved by measuring the resonant frequency shift of the microwave resonators (green). (c) Spectroscopy of $Q_A$ as a function of the external flux through the qubit loop. Lines indicate a fit to the spectrum of the qubit model. (d) Energy diagrams of the qubit-resonator coupled system for $Q_A$ at the ${\mathrm{\Phi }}_{\mathrm{ext}}^A=0.5{\mathrm{\Phi }}_0$ flux sweet spot and ${\mathrm{\Phi }}_{\mathrm{ext}}^A=0.425{\mathrm{\Phi }}_0$ where the reset is performed. Microwave irradiations used for qubit excitation (blue), reset (red), and readout (green) are shown as arrows. Dashed lines indicate levels without qubit-resonator interaction. (e) two-qubit spectroscopy with the transition frequency of $Q_A$ sweeping across that of $Q_B$ idled at the flux sweet spot.

Figure 2

(a) Sequence fidelities of reference and interleaved RB with sequences of $m$ random Clifford gates. For the fidelity of each value of $m$, 20 random sequences are used to obtain the average sequence fidelity, and the standard deviation is displayed as error bars. (b) Fidelity of $X_{\pi /2}$ as a function of gate duration. The black dashed line represents the calculated coherence limit.

Figure 3

(a) Schematic of the level structure involving the energy eigenstates $|0_A{1}_B⟩$ and $|1_A{0}_B⟩$. Insert: the flux control pulse that activates the iSWAP operation. (b) Coherent oscillation of the population between the state $|0_A{1}_B⟩$ and $|1_A{0}_B⟩$, shown as $P_{01}$ as a function of the pulse amplitude ${\mathrm{\Phi }}_{\mathrm{amp}}$ and pulse length $t_g$. The magenta dashed line corresponds to the pulse amplitude where the swapping rate is at the minimum and the system undergoes a complete population exchange from one qubit to the other. (c) Sequence fidelity of Clifford RB and interleaved RB of the iSWAP gate. Each fidelity is obtained from averaging over 10 random sequences. (d) Temporal fluctuation of the fidelities for the two-qubit Clifford gates and the iSWAP gate. The error bars are calculated from the uncertainty of fitting parameters.